1. Field of the Invention
The present invention relates to a magneto-optical recording method, and more particularly to a magneto-optical recording method capable of performing overwriting.
2. Discussion of Background
In recent years, the development of magneto-optical recording has proceeded actively because this method is capable of recording higher volumes of information than conventional magnetic recording and the like, and because rewriting of the information is possible. Some of these developments are already on the market. However, with many of the magneto-optical recording apparatus now on the market, when information is written, it is necessary to first erase the original information, then write in the new information. Considerable time is wasted in making this erasure.
Although many different magneto-optical recording methods in which overwriting is possible have been proposed, they have not been used in practice because of a number of problems.
The following magneto-optical recording methods are conventionally known:
(1) A general light modulation system (for example, described in Applied Magnetics Association Journal Vol. 8. No. 5 (1984)). PA0 (2) An overwrite system by varying light intensity using a two-layered film (for example, described in Applied Magnetics Research Association Data 55-15 (1988)). PA0 (3) A magnetic field modulation system utilizing a fixed magnet (for example, described in Electrical Society Magnetics Research Association Data MAG-86-96)). PA0 (4) A magnetic field modulation system utilizing a floating head (for example, described in Electrical Society Magnetics Research Association Data MAG-87-178(1987), Japanese Laid-Open Patent Application 63-204532 and Japanese Laid-Open Patent Application 63-217548)). PA0 (5) An overwrite system utilizing a magnetic head with a resonance circuit and a pulsed laser (for example, described in IEEE Trans. Magn. 24, P.666 (1988) and Japanese Laid-Open Patent Application 63-37842). PA0 (6) A system utilizing demagnetizing field (Han-Ping D. Shieh and Mark H. Kryder, Appl. Phys. Lett. 49(1985) 473; Han-Ping D. Shieh and Mark H. Kryder, IEEE Trans. Magn., Vol. MAG-23(1987)171; M. D. Schultz, H-P.D. Shieth and M. H. Kryder, J. Appl. Phys. 63 (1988)3844).
An analysis of the systems listed above shows that: with the light modulation system (1), it is generally impossible to overwrite unless an erasure operation is first performed, with the attendant problem that considerable time is required for rewriting.
With the overwrite system (2) by varying light intensity, using a two-layered film, the recording layer becomes complex in structure, so that the fine adjustment of the exchange bonding between the two layers is very difficult, and a large magnetic field is necessary for initialization. The problem therefore arises that a large expensive magnet must be used for initialization so that the size of the apparatus increases. Furthermore, the difference in power between the light beams with "1" and "0" during recording is so small that the respective allowable differences are restricted, so that the recording becomes unstable.
With the magnetic field modulation system (3), utilizing a fixed magnet, a large magnet is required and the amount of power consumed is therefore large. It is also extremely difficult to conduct high speed recording, and it is generally agreed that this system is restricted to digital recording at about 1 MHz.
With the magnetic field modulation system (4), utilizing a floating head, the magnetic head almost touches the optical recording disk medium so that the advantage of avoiding contact with the optical disk medium is essentially lost. Furthermore, there is the necessity for forming the medium in the form of a single plate, which causes problems with the volume of memory and in protection of the substrate and the magnetic layer formed thereon.
The overwrite system (5) utilizing a magnetic head with a resonance circuit and a pulsed laser is better than the above-mentioned magnetic field modulation system (3), but exhibits the same type of problems.
With the system (6) utilizing demagnetizing field, the laser pulse must be precisely directed at the position of the previously recorded bit, therefore control of positioning of the laser pulse is difficult. Another problem is the fact that recording can take place only at these bit positions.